The present invention relates to a turbine element having a cast-in bump-covered cooling surface on one side and a coating overlying the cast-in bump surface to provide improved heat transfer between a coolant and the backside of the turbine element. The present invention also relates to methods for improving the heat transfer between the coolant and a turbine element having a cast-in bump-covered surface on one side.
Various techniques have been devised to maintain the temperature of turbine engine components below critical levels. As an example, a cooling medium, such as coolant air from the engine compressor is often directed to the component, along one or more component surfaces. Such flow is understood in the art as xe2x80x9cbackside air flow,xe2x80x9d where coolant air is directed at a surface of an engine component that is not directly exposed to high temperature gases from combustion. In combination with backside air flow, xe2x80x9cturbulatorsxe2x80x9d have been used to enhance heat transfer. Turbulators are protuberances or xe2x80x9cbumpsxe2x80x9d on selected sections of the surface of the component, which function to increase the heat transfer between the cooling medium and the turbine element.
An example of the use of turbulators to enhance heat transfer is found on the impingement cooling side of a shroud encompassing the hot gas path of a turbine. It will be appreciated that the outer shroud of a gas turbine surrounds the hot gas path and is subject to very high temperatures on the hot gas path exposed side thereof. A cooling medium is conventionally disposed on the opposite side of the shroud from the hot gas path to cool the shroud wall. For example, in advanced turbine designs, the cooling medium may comprise steam which is directed onto the coolant side of the shroud wall through an impingement plate. The coolant side of the shroud wall has cast-in bumps generally cylindrical in shape and spaced from one another which provide a coolant side surface area which is larger than that of the base line smooth surface area. For example, the coolant side surface area ratio may be about 1.2. That is, the surface area of the coolant side of the surface with the cast-in bumps may be 1.2 times larger than the surface area of the coolant side of the element without the cast-in bumps. Improved heat transfer values, for example, 1.15 for jet Reynolds numbers ranging from 10,000 to 40,000 have been demonstrated. Heat transfer enhancement value is defined as the ratio of heat transfer from a surface with cast-in bumps to the heat transfer from a smooth surface.
It has been found that the processes employed for casting the bumps onto the surface of the element limit the dimensions of the cast-in bump geometries and the inter-bump spacing or pitch. As a consequence of these manufacturing process limitations, the heat transfer enhancement values for cast-in bump surfaces are limited. For example, it has been found very difficult to increase the heat transfer enhancement ratio beyond 1.4 using cast-in bumps on the coolant side of the element. Also, certain areas on the element are not amenable to receiving cast-in bumps, e.g., due to liquid metal filling issues and bump mold removal problems. Those areas may include mold joints, seams and bare spots for indexing/locating pins. Consequently, there is a need to provide enhanced heat transfer characteristics beyond those afforded by cast-in bumps on the coolant side of the element.
In accordance with a preferred embodiment of the present invention, cooling enhancement material is applied to the cooling side of the element in overlying relation to the cast-in bumps and the spaces between the bumps. Preferably, a coating containing particles, e.g., formed of metal particles, is applied to the cooling surface of the element overlying the cast-in bumps and spaces therebetween. Preferably, a green braze tape coated with a metallic powder is set in intimate contact with the cooling surface with cast-in bumps and then brazed in a vacuum oven. The size of the metallic powder particles is selected to provide a heat transfer enhancement ratio larger than that provided by the bumps per se. For example, metallic particle sizes in a range of 1 to 20 mils may be used. A coating formed with metallic particles having diameters of 15 mils has resulted in impingement heat transfer enhancement values from 1.3 to 1.8 for the range of Reynolds numbers between about 10,000 and 50,000. With the brazed rough coating of cooling enhancement material applied to the coolant side of the element, the increased impingement heat transfer can be used to reduce the metal temperatures of the element and increase its expected life; to increase the hot gas side temperatures, hence increasing overall efficiency of the turbine; and reduced cooling flow requirements, hence reducing compressor discharge air used for cooling and increasing efficiency. It will also be appreciated that the coating with the cooling enhancement material does not significantly increase the pressure drop in comparison with the pressure drop with cast-in bump surfaces alone and, consequently, there is no significant pressure drop penalty for the use of brazed microturbulators. One method of increasing heat transfer characteristics in this embodiment is described in co-pending U.S. patent application Ser. No. 09/304,276, filed May 3, 1999, of common assignee herewith.
In a preferred embodiment according to the present invention, there is provided turbine component comprising an element separating a high-temperature region and a cooling medium from one another, the element on a cooling medium side thereof having a surface with discrete bumps separated from one another and projecting from the surface, the surface with the discrete bumps defining a predetermined ratio of the area of the surface with the bumps and the area of the surface without the bumps, a surface coating on the cooling side of the element overlying the surface with discrete bumps forming a cooling side surface having a ratio of the area of the coated surface with the bumps and the area of the surface with the bumps without the coating in excess of the predetermined surface area ratio to afford increased heat transfer between the cooling medium and the element relative to the heat transfer between the cooling medium and element without the coating.
In a further preferred embodiment according to the present invention, there is provided a method of enhancing the heat transfer between an element having a surface with cast bumps projecting from the surface and a cooling medium, the surface with the cast bumps defining a predetermined surface area ratio comprising the steps of applying a coating on the surface to overlie the cast bumps and areas on the surface between the cast bumps to form a coated surface having a surface area ratio in excess of the predetermined surface area ratio to afford increased heat transfer between the element and the cooling medium relative to the heat transfer between the element and the cooling medium without the coating.
In a still further preferred embodiment according to the present invention, there is provided a method of enhancing the heat transfer between an element having a surface with cast bumps projecting from the surface and a cooling medium, comprising the steps of providing a brazing sheet having cooling enhancement material and fusing the brazing sheet to the surface including the cast bumps such that the cooling enhancement material is bonded to the surface.